1) Field of the Invention
The present invention relates to an image processor and an image processing method, which are applied to image forming apparatus such as printers, digital copying machines, and facsimiles, using an electrophotographic process, and apparatus having the similar function, and performs the optimum image processing, particularly, filtering processing for correcting the sharpness, by using the attribute information of an image.
The present invention relates to an image processor, which is applied to an image processor such as digital copying machines and facsimiles using the electrophotographic process, and particularly, applied to an image processor that detects character edges in an input image, for improving the image quality, and performs appropriate processing based on the detection result.
2) Description of the Related Art
In the image processor such as the digital copying machine and the facsimile, heretofore, the image attribute information such as characters and non-characters (segmentation result) is obtained from the input image signal, and the spatial frequency characteristic of a filter is controlled, based on the obtained image attribute information.
(1) Japanese Patent Application Laid-Open No. 7-95409 discloses an example in which an attribute is judged as a character or a non-character for each pixel unit by a segmentation circuit, and a strong edge enhancement filter is applied to character pixels, and a relatively weak filter is applied to non-character pixels based on the judgment result.
(2) In the image processing method disclosed in Japanese Patent Application Laid-Open No. 2000-134625, an image signal is subjected to sub-band processing, the band components are corrected by referring to the divided components, and the corrected band components are synthesized to form an image signal, to thereby remove noise and emphasize the edge.
(3) In the image processing method disclosed in Japanese Patent Application Laid-Open No. 6-274614, wavelet transform is performed with respect to an image signal, to resolve it into signals in a plurality of frequency bands, and predetermined image processing inverse wavelet transform is performed with respect to at least one frequency band signal. In this predetermined image transform, processing for multiplying a signal in at least one frequency band by a predetermined number is carried out. As a result, a naturally reproduced image corresponding to visual impressions can be obtained, without emphasizing noise at the time of image reproduction.
In the inventions (2) and (3), sharpness is controlled for each band and for each direction, by band-dividing an image signal and correcting the obtained band components to perform inverse transform. The invention (2) is proposes to divide the image signal into a plurality of bands by an optional band dividing filter, and the invention (3) proposes to performing band division by wavelet transform.
(4) The image processing method and the image processor disclosed in Japanese Patent Application Laid-Open No. 6-223172 generate a plurality of band signals from image data, and extracting edge information of the image data by at least one band signal, to detect a character area from the distribution of edge information.
The image quality in the image processor such as digital copying machines and facsimiles can be improved by smoothing the images. When a portion where halftone is expressed by a halftone dot on an original document is read and processed, smoothing for pattern processing is necessary, in order to prevent moire due to halftone dots. On the other hand, if smoothing for pattern processing is similarly carried out with respect to characters, other than characters on a white ground such as characters on dots, sharpness is insufficient, thereby the image quality is deteriorated.
Taking the ground of the character into consideration, conventional techniques for improving the image quality include the followings.
(5) The invention disclosed in Japanese Patent Application Laid-Open No. 8-181864 uses a dot degree calculating filter, which responds to the dot portion in the input image, to perform smoothing processing according to the result. A method is proposed herein, which calculates the degree of dot from the input image, and perform image correction corresponding to the degree of dot, thereby enabling adequate image correction, respectively, with respect to an original document in which all sorts exist together, for example, a character on white ground, a character on dots, dots, and a picture.
(6) The invention disclosed in Japanese Patent Application Laid-Open No. 6-223172 has a configuration such that a plurality of band signals are generated from image data, and edge information of image data is extracted by at least one band signal, to detect a character area from the distribution of edge information. In this invention, a configuration in which a density of band signals having a larger value than a certain threshold is detected to carry out character judgment is disclosed.
However, in the technique shown in (1), since strong and weak filters are switched and used based on the image attribute information judged digitally, the strength of the filter abruptly changes at a boundary portion between the character area and the non-character area. There will be no problem when the judgment of character and non-character areas can be performed highly accurately. However, if a dot portion having a relatively low number of lines, and a pixel, which is misjudged to be a character due to edges in a pattern image, exist together, a strong emphasis filter is locally applied to the pixel.
As a result, pixels having high density scatter in the dot image, causing a problem in that graininess deteriorates, edges of hair and eyebrow in the pattern are emphasized extremely strongly, to give the image a sense of incompatibility, or defects due to a segmentation error are noticeable. In order to solve the problem in such a binary switching, there is an invention in which multivalued segmentation result is generated and stored, to control the filter in multiple steps. However, memory for storing the segmentation data increases, thereby causing a problem of a cost increase.
In the inventions described in (2) and (3), adaptive processing using the image attribute information such as the image area segmentation result is not performed, but a similar control is performed with respect to the character image and the dot image. Therefore, sharpness of the character image and moire suppression in a dot image cannot coexist.
In the invention described in (1), a feature of a character is extracted from the divided signal by an area dividing unit such as wavelet transform, but it relates only to extraction of the feature of a character, and there is no technical disclosure regarding the filter processing using the feature of a character.
In the technique described in (5), the dot detection processing is necessary separately, in order to obtain sharpness satisfactory for a character on dots having a different characteristic from that of a character on white ground. A large processing load is required for such detection processing of the dot portion.
Even with the technique disclosed in (6), a band signal having a large value exists in a dot image, and hence the accuracy of character judgment cannot be improved. Since the configuration is such that a low frequency band signal is generated, while performing downsampling, the judgment accuracy is poor.
As a method for solving the problems other than the conventional methods, there is a method in which an edge that is not a dot, that is, only a character edge is extracted to perform sharpness processing based on the extraction result. According to this method, the character itself on dots is emphasized, and the sharpness is improved. However, since dots adjacent to the character on dots are also emphasized, unevenness that does not exist in a character is reproduced, and as a whole, the image quality of the character on dots cannot be improved.